(a) Field of the Invention
The present invention relates to a rechargeable lithium battery and a method of manufacturing the same. More particularly, the present invention relates to a rechargeable lithium battery that has an irreversible capacity during a first charge and discharge, and has no problems such as dendrites, electrolyte decomposition, or dissolution of a negative current collector.
(b) Description of the Related Art
In general, a material that is reversibly capable of intercalating and deintercalating lithium ions has been used for an active material for a rechargeable lithium battery. For a positive active material of a rechargeable lithium battery, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), and so on has been used. For a negative active material of a rechargeable lithium battery, a carbon material has been used. Examples of the carbon material are graphite, amorphous carbon, or low-crystalline carbon.
Lithiums included in a positive active material are transferred between a positive electrode and a negative electrode in the form of lithium ions during charge and discharge and reversibly intercalated into or deintercalated from a positive active material or a negative active material.
Carbon material such as graphite, amorphous carbon, or low-crystalline carbon has a large charge and discharge capacity, but also has an irreversible capacity. The irreversible capacity refers to a capacity incurred by lithium ions that are not deintercalated from the carbon material at a second or subsequent discharge after they are intercalated into the carbon material at a first charge, and thus do not participate in a cell reaction. Particularly, an irreversible capacity of the carbon material at first charge is up to 5% to 10% of the total charge and discharge capacity of the battery.
However, when a part of lithium supplied from a positive active material to a negative active material at an initial charge can not be restored during the subsequent discharge, the amount of lithium ions become smaller and thereby contribute less to the charges and discharges after the first one, failing in acquiring a rechargeable lithium battery with high-capacity. Here, even though a carbon material with big charge and discharge capacity is used, the result can be the same when irreversible capacity is too big. In addition, the irreversible capacity can be generated by a Si-based material, a Sn-based material, or an Al-based material, as well as the carbon material.
However, since the carbon material has extremely smaller irreversible capacity from the second charge and discharge reaction, there is no more worry about sharp decrease of the amount of lithium ions contributing to the charge and discharge reaction. Accordingly, in order to accomplish high-capacity of a rechargeable lithium battery, there have been suggestions such as reforming a carbon material, decreasing irreversible capacity itself at the initial charge, or charging the amount of lithium corresponding to the irreversible capacity inside a battery before the charge.
In particular, the last one has been paid more attention to, in which the same amount of metal lithium as irreversible capacity is included in a negative electrode before the reaction, so that it can be used as an active material at the first discharge and compensate the lithium corresponding to irreversible capacity.
In addition, there is another suggestion that a rechargeable lithium battery is fabricated by charging a negative electrode with lithium-containing composite nitride instead of metal lithium with a carbon material (Japanese Patent laid-open No. 2002-117836).
However, in a rechargeable lithium battery including a negative electrode pre-charged with metal lithium, since it is hard to predict the exact amount of metal lithium corresponding to irreversible capacity, it has been designed to include a little more amount of metal lithium than the exact amount corresponding to irreversible capacity. Accordingly, a rechargeable lithium battery designed like the above could include a negative electrode containing the residue of metal lithium after the initial discharge.
Lithium ions can be deposited from the surface of the metal lithium at the next charge. In addition, lithium ions are generally deposited as a dendrite shape, which can penetrate a separator, and thereby generate a short-circuit between the negative and positive electrodes.
Furthermore, the lithium-containing composite nitride disclosed in Japanese Patent laid-open No. 2002-117836 can be decomposed while reacting with water. Accordingly, when a negative electrode including the lithium-containing composite nitride is fabricated, its atmosphere should be dry, which results in increasing manufacturing cost.
On the other hand, FIG. 2 of Japanese Patent laid-open No. 2002-117836 disclosed a charge and discharge curved line of a rechargeable lithium battery including LiCoO2 as a positive electrode and a carbon material and lithium-containing composite nitride as a negative electrode. A shown in FIG. 2, the lithium-containing composite nitride had discharge reaction at a voltage range of 2.5 to 3V. However, the charge and discharge curved line was gained by only regulating a charge and discharge current and a cut-off voltage at the charge and discharge but did not reflect a method of regulating the amount of discharge against the amount of initial charge. Accordingly, the rechargeable lithium battery is over-charged with lithium-containing composite nitride compared with irreversible capacity at the first charge and discharge, and thereby, over-discharged fro such a long time, which leads to decomposition of an electrolyte solution and dissolution of a negative current collector.